The Giant Magneto-Resistance (GMR) effect was discovered in ultra thin magnetic multilayer films in 1988. The GMR multilayer structure is composed of alternating ferromagnetic and nonmagnetic metal layers. The origin of the GMR effect is due to the spin-dependent scattering in thin magnetic multilayers with thickness of a few nanometers. At the zero-field state, magnetizations of adjacent ferromagnetic layers are antiparallel due to Ruderman, Kittel, Kasuya, and Yoshida (RKKY) coupling between layers. However, when an external magnetic field is applied, all magnetizations will rotate toward the field direction, resulting in a decrease of magnetoresistance due to less spin scattering.
The lowest magnetoresistance occurs at the state when the magnetizations of adjacent ferromagnetic layers are aligned parallel by the applied magnetic field. GMR sensing elements based on GMR multilayers have many advantages over other magnetic sensing elements with regard to size, simplicity, power consumption, sensitivity, frequency characteristics and thermal stability. As a result, the GMR sensing element has been applied in various sensors. For example, GMR sensing elements have been used in magnetic heads, position sensors, magnetic encoders, gear tooth sensors and other applications.
The most difficult technical challenges in GMR sensor applications remain: 1) fabricating the GMR multilayers with desired quality; and 2) fabricating GMR sensors with low hysteresis, excellent linearity and desired sensitivity.